We now have circular dichroism (CD) spectra for 12 synthetic DNA polymers of defined, simple sequences. These 12 spectra can be combinnd into 65 different sets of 8 spectra each. Each set can then be used to obtain computerized fits to the spectra of natural satellite and bacterial DNAs, if a CD spectrum is considered to be a nearest-neighbor property. From such spectral analyses, estimates of nearest-neighbor frequencies can be obtained for the natural DNAs. We propose to use such analyses to find which synthetic polymers are best or least suitable as models for a given natural DNA. This procedure should indicate whether any of the synthetic DNAs or satelitte DNAs are in unusual geometries, compared with DNAs of complex sequences. In addition, we will compare the CD spectra and conformations of linear versus covalently-closed natural DNAs under a number of solution conditions. Both forms of DNA exist as coiled-coils, as supercoils, when condensed by ethanol dehydration and completely dried on electron microscope grids. So far, we have found that both DNA forms do act similarly to ethanol dehydration in solution, but the assignment of a given CD spectrum to a supercoiled tertiary form remains to be done. BIBLIOGRAPHIC REFERENCES: Ratliff, R. L., Hoard, D. E., Hayes, F. N., Smith, D. A., and Gray, D. M. "Preparation and Properties of the Repeating Sequences Polymers d(A-I-C)n.d(I-C-T)n and d(A-G-C)n. d(G-C-T)n." Biochemistry (1976) 15, 168-176. Lang, D., Taylor, T. N., Dobyan, D. C., and Gray, D. M. "Dehydrated Circular DNA: Electron Microscopy of Ethanol-Condensed Molecules." J. Mol. Biol., in press, (1976).